The present invention relates to molybdenum which is coated with a noble metal and to a method for producing such a layer sequence.
Such layer sequences are frequently used in the semiconductor art as contact materials because the coefficient of thermal expansion of the silicon used as the semiconductor material and the coefficient of thermal expansion of the molybdenum used as the contact material are very similar to one another and thus produce no, or only slight, thermal stresses under changing operating temperatures. Customarily the molybdenum which is used, for example, in the form of discs having a disc thickness of about 0.5 to several mm, is coated with a noble metal which produces a favorable electrical contact resistance. On the other hand, the noble metal protects the molybdenum contact against surface oxidation and the attack of aggressive chemicals which act on the semiconductor body in the course of further process steps incorporating etching treatments. Such chemicals are required to shape the semiconductor body and give it its particular properties, but most not damage the molybdenum contact during such etching treatments.
Due to the high price of noble metals, it is desirable to have the coating as thin as possible, but a certain minimum thickness of the noble metal must be provided because otherwise sufficient etching resistance is no longer assured. For example, with gold as the coating material, a satisfactory etching resistance is achieved only if the layer thickness is at least 1.5 to 3.mu.. With a layer thickness less than 1.5.mu., pores develop through which the attack of the molybdenum base material during etching cannot be avoided. For that reason, it has previously been necessary in practice to provide a thicker gold layer. Additionally, thinner layer thicknesses for the coating material result in higher costs for the base material because it must be prepared better and more uniformly.
Moreover, the high mechanical sensitivity of such noble metal thin layers is a drawback. If, for example, circular discs are to be cut from the gold-coated molybdenum sheets, which is advisable and the general custom, this mechanical sensitivity requires noble metal layer thicknesses of about 5.mu.. With small disc diameters, for example, diameters of about 6 mm, the noble metal layers must be up to 10.mu. thick.
Experiments have been made to replace noble metals, such as gold, by other more economically priced metals. However, almost all non-noble metals have a much too low etching resistance and are unsuitable already for this reason. The only suitable non-noble metal seems to be chromium which is vapor-deposited or applied electrochemically and exhibits good adhesion as well as good etching resistance.
However, the drawback of a chromium coating is that it produces poor and, most importantly, fluctuating transfer resistances which preclude general application. These fluctuating transfer resistances of chromium are probably caused by oxidation processes at the surface which occur randomly and irregularly and cannot be influenced directly. In the course of the manufacture of a semiconductor component for which molybdenum discs are used, several process steps involve temperatures of more than 100.degree. C. which obviously enhances the above-mentioned oxidation processes. These process steps which involve temperatures of more than 100.degree. C. occur from the time the diffused silicon body is alloyed onto the molybdenum disc, which is coated with chromium on one side, until the assembly is installed in a hermetically sealed housing.
The poor fluctuating transfer resistances are observed mainly when the chromium is deposited electrochemically, but can also not be avoided if the chromium is vapor deposited. Additionally, coating by way of vapor deposition requires a complicated and much more expensive procedure. The advantage of chromium, which is less expensive than the noble metal, is more than cancelled out of by the uneconomical coating process. For the above reasons, chromium has also failed to find acceptance in practice as a coating material and is unable to replace the conventional thicker noble metal layers.